In electromagnetic prospecting, a coil or a loop carrying a pulsed or alternating current causes induced currents (I in FIG. 1A) to flow in conducting ore bodies, and the time-varying magnetic field (also called secondary magnetic field) associated to these currents is detected by means of a second coil (or receiver loop) placed some distance away from the first one. A mapping of this secondary magnetic field may thus give information on the distribution of conductors in the ground under exploration.
Such time domain electromagnetic surveys can be performed either using a moving transmitter (moving loop surveys) or a fixed transmitter loop (fixed loop surveys). In the case of moving loop surveys, the transmitter loop and receiver loop setup is moved along survey lines for every reading in order to produce a map of buried conductors under the investigated area. In the case of fixed loop surveys, a fixed transmitter single-loop 6, deployed on the ground as illustrated in FIG. 1A, produced a time-varying primary magnetic field and a roving receiver sensor measures, along survey lines 13, the secondary magnetic field produced by the conductors in the ground.
When the survey lines are located outside the transmitter loop, the configuration is called an out-of-loop survey. When the survey lines are located inside the transmitter loop, the configuration is called an in-loop survey. A given survey may naturally combine the out-of-loop and in-loop surveys.
In the search of steep-dipping geological conductors, the main advantages of the out-of-loop survey are the uniformity and the sub-horizontality of the primary electromagnetic field under the survey area. This is illustrated in FIG. 2A and FIG. 2B where the direction of the magnetic field 33′ in the ground under the transmitter loop 6 of FIG. 1A has been drawn (FIG. 2A) as well as its intensity 34′ (FIG. 2B). It can be seen from FIG. 2A that the primary electromagnetic field under the out-of-loop area is quite horizontal. This optimizes the coupling between the primary electromagnetic field and any buried steeply-dipping conductor. However, the main disadvantage of the out-of-loop configuration is the exponential decrease of the intensity of this primary electromagnetic field from the loop side, as illustrated in FIG. 2B. The sensitivity and the effectiveness of the configuration decrease with distance from the loop side.
For the same type of geological conductor, the main advantage of the in-loop survey is the strong intensity of the primary electromagnetic field under the loop, as can be noted on FIG. 2B. This intensity permits better sensitivities and greater depths of investigation. The main disadvantage of the in-loop mode is the sub-vertical orientation of the primary electromagnetic field under the loop, as illustrated in FIG. 2A. This orientation of the primary magnetic field causes a poor coupling between the primary electromagnetic field and any sub-vertical conductors.
A dual-loop configuration, with two subset loops 7, 8 connected in parallel, as illustrated in FIG. 1B, has been proposed for mobile loop surveys to create a sub-horizontal primary magnetic field under an in-loop zone. However, the primary magnetic field produced by this loop configuration lacks uniformity. Another drawback of this dual-loop configuration is that since the two subset loops are connected in parallel, the current in each subset loop (currents I1, and I2) may be slightly different and this can create some parasitical effects. Also note that moving the mobile loop is complicated and unproductive.
Thus, there is a need for a transmitter loop configuration that would provide at the same time an intense, substantially uniform and substantially horizontal primary electromagnetic field without the foregoing disadvantages.